At present, a discharge type fluorescent lamp and an incandescent bulb used as the illumination device involve problems that a harmful substance such as mercury is contained, and life span is short. However, in recent years, a high luminescence LED emitting light of near ultraviolet/ultraviolet to blue color has been developed in sequence, and the white LED illumination for the practical application of the next generation has been actively studied and developed, in which the white light is prepared by mixing the light of the near ultraviolet/ultraviolet to blue color generated from the LED and the light generated from the phosphor having an excitation band in a wavelength region thereof. When the white LED illumination is put to practical use, since efficiency of converting electric energy into light is improved, less heat is generated and it is constituted of the LED and a phosphor, the white LED has advantages of good life span without burn-out of a filament like a conventional incandescent bulb and the harmful substance such as mercury is not contained, and miniaturization of the illumination device is realized, thus realizing an ideal illumination device.
At present, two systems of the white LED illumination are proposed. As one of them, a multi chip type system is given, in which three primary color LEDs such as red LED, blue LED, and green LED with high luminance are used. As the other of them, one chip system is given, in which LED with high luminance emitting in near ultraviolet/ultraviolet to blue color and the phosphor excited by the light having an emission spectrum with a peak in the range from ultraviolet to blue color generated by the LED are combined. When such two systems are compared from the viewpoint of illumination, particularly in the one chip system, by using the phosphor having a broad emission spectrum, the white emission spectrum is approximated the spectrum of the sun-light, and the color rendering properties are possibly improved. Further, the one chip system has a lot of advantages such that a drive circuit can be simplified and small-sized, a light guide for performing color mixture is unnecessary, and a difference in a drive voltage and light output of each LED and temperature characteristics need not to be considered, and cost can be reduced. This contributes to focusing on the one chip system as the illumination of next generation, compared with the multi chip type system. The white LED illumination, in which the high luminance blue LED and the phosphor emitting yellow color by being excited by the blue light generated from the LED are combined, is given as one of the examples of the one chip type while LED illumination. Specifically, for example, the high luminance blue LED and the yellow phosphor (Y, Gd)3, (Al, Ga)5O12:Ce (YAG :Ce), Tb3Al5O12:Ce, Ca3Si3O12:Ce can be combined. In the white LED illumination, white color is obtained by using a complementary relation between the blue emission of the LED and yellow emission of the phosphor, thereby allowing fewer phosphors to be used. Further, the yellow phosphor YAG :Ce used for the white LED illumination has an excitation spectrum with a peak near the wavelength of 460 nm, thereby allowing emission with high efficiency, and has an emission spectrum with a luminance (visibility) peak at about 560 nm, thereby allowing high luminance white LED to be obtained. However, the problem of the white LED illumination is that the emission on the long-wavelength side of visible light range, specifically the emission of red color component is insufficient, and therefore, only slightly bluish white emission can be obtained, and a slightly reddish white emission like an electric bulb can not be obtained, thereby deteriorating in the color rendering properties. However, in recent years, the phosphor having an excitation spectrum with a peak in the wavelength range from yellow color to red color, and having an emission spectrum with a peak in a broad range, and also having a good excitation band in a range from near ultraviolet/ultraviolet to blue color has been developed in sequence. Then, by adding such a phosphor, the color rendering properties are improved.
Also, the white color LED illumination in which white color is obtained by using a mixed state of colors of the lights of the LED emitting the near ultraviolet/ultraviolet color, and the phosphor emitting red color (R), the phosphor emitting green color (G), and the phosphor emitting blue (B) color obtained by being excited by the near ultraviolet/ultraviolet light generated from the LED, is given as another example of the one chip type white LED illumination. A method of obtaining white emission by the lights of the R, G, B, and other colors is capable of obtaining an arbitrary emission color other than white light, depending on the combination and mixed ratio of the R, G, B, and is excellent in color rendering properties, because the white emission is obtained not by the complementary relation of the light but by the relation of mixed state of colors using the R,G,B.
Then, as the phosphor used for such an application, examples are given such as Y2O2S:Eu, La2O2S:Eu, 3.5MgO.0.5MgF2.GeO2:Mn, (La, Mn, Sm)2O2S.Ga2O3:Eu for the red phosphor, ZnS:Cu, Al, CaGa2S4:Eu, SrGa2S4:Eu, BaGa2S4:Eu SrAl2O4:Eu, BAM:Eu, Mn, Ba2SiO4:Eu for the green phosphor, and BAM:Eu, Sr5(PO4)3Cl:Eu, ZnS:Ag, (Sr, Ca, Ba, Mg)10(PO4)6Cl2:Eu for the blue phosphor. However, the red phosphor out of the phosphors of three colors has a sharp emission spectrum, while the phosphors of other colors have spectra with peaks in a broad range, thereby involving the problem that the color rendering properties of the white light obtained is unsatisfactory, and emission characteristic at a high temperature is deteriorated. However, such a problem has also been solved, as described above, by developing in sequence the phosphors containing nitrogen, excellent in temperature characteristic and excitation band characteristic, and emitting from yellow color to red color.
The problem involved in the phosphor emitting yellow color to red color is substantially solved, by developing the phosphor having the emission spectrum with a peak in the wavelength range from yellow color to red color, having a broad emission spectrum, and further having a good excitation band in the wavelength range from the near ultraviolet/ultraviolet to blue color. As the phosphor containing nitrogen as described above, Ca2Si5N8:Eu, Sr2Si5N8:Eu, Ba2Si5N8:Eu, Cax(Al,Si)12(O,N)16:Eu(0<x≦1.5), CaAl2Si4N8:Eu, CaSiN2:Eu CaAlSiN3:Eu and so forth are typically given as examples.
By developing a new phosphor as described above emitting from yellow color to red color, the problem is involved in sequence in the phosphor having the emission spectrum with a peak in the wavelength range from green color to yellow color.
First, the problem involved in the yellow phosphor YAG :Ce is explained by using FIG. 15. FIG. 15 is a view showing an excitation spectrum obtained by measuring an intensity of the light with the wavelength of 559.2 nm emitting light when the YAG :Ce is excited by an excitation light with the wavelength of 300 to 570 nm.
In the white LED illumination obtained by combining the high luminance blue LED and the YAG :Ce phosphor emitting yellow color by being excited by blue color generated from the LED, the YAG :Ce phosphor has a high efficient excitation band for the light with the wavelength of 460 nm generated from the blue LED, and further, has an emission spectrum with a luminance (visibility) peak at closest to the wavelength of 560 nm or around, thereby allowing a high luminance white LED Illumination to be obtained. However, as clarified from FIG. 15, the YAG :Ce phosphor has an emission characteristic of emitting the light with the wavelength of 560 nm or around with high efficiency, when excited by the light with the wavelength of 460 nm. However, the emission wavelength of the blue LED is changed due to variation in manufacturing the blue LED when excited by the blue light of the blue LED. Then if the emission wavelength is deviated from the range of an optimal excitation band of the YAG :Ce, disruption of balance between the blue color and yellow color emission intensity occurs because of the narrow excitation band of YAG :Ce. Such a situation involves the problem that color tone of the white light obtained by synthesizing the blue light and the yellow light is changed.
Further, the YAG :Ce phosphor has an excellent emission spectrum in the wavelength range from about 500 to 550 nm of green color component of visible light. Therefore, preferably the YAG :Ce phosphor is used as a green phosphor of the white LED illumination in which the near ultravilet/ultraviolet LED, the red (R) color emitting phosphor, the green (G) color emitting phosphor, and the blue color (B) emitting phosphor are combined. However, when emitted by the near ultraviolet/ultraviolet light, as shown in FIG. 15, the YAG :Ce phosphor has a low efficient excitation band in the emission wavelength of 380 to 410 nm or around of the near ultraviolet/ultraviolet LED. Therefore, the problem involved therein is that a sufficient emission can not be obtained, and the high luminance white LED illumination can not be obtained.
Next, the problem involved in the green phosphor used in the ultraviolet emission will be explained. As the white LED illumination using the light in a mixed state of the near ultraviolet/ultraviolet emitting LED and the red (R) color emitting phosphor, the green (G) color emitting phosphor, and the blue (B) color emitting phosphor obtained by being excited by the light of the near ultraviolet/ultraviolet light generated from the LED, at present, the green phosphor such as ZnS:Cu, Al, SrAl2O4:Eu, BAM:Eu, Mn, Ba2SiO4:Eu are used. Out of such phosphors, the problem is that a sulfide phosphor is significantly deteriorated in emission intensity, when heat is applied thereon, and further has no water-resisting property. In addition, an oxide phosphor does not have a good efficient excitation band in a broad range of the wavelength in the vicinity of the near ultraviolet/ultraviolet. Therefore, the problem involved therein is that when the variation in emission wavelength occurs due to by variation in manufacturing the near ultraviolet/ultraviolet LED, the emission wavelength of the near ultraviolet/ultraviolet LED is deviated from the optimal excitation range, thereby disrupting the balance in emission intensity among the red color, green color, and blue color, resulting in the change of the color tone of the white light.
Therefore, as the green to yellow emitting phosphor by being excited by the light of the near ultraviolet/ultraviolet to blue color also, demand on the new phosphor having a flat high efficient excitation band in the wavelength range from the near ultraviolet/ultraviolet to blue color, and having a broad emission spectrum, and further having an excellent durability against heat and water, and replacing the YAG :Ce phosphor and the ZnS:Cu, Al phosphor is increased. In order to respond to such a demand, the green to yellow emitting phosphor is actively pursued, and in recent years, silicon nitride-based phosphor (for example see patent document 1), and oxynitride phosphor (for example, see patent documents 5 and 6) are proposed as the green to yellow emitting phosphor. (Patent document 1) Japanese Patent Laid Open No.2002-322474 (Patent document 2) Japanese Patent Laid Open No.2003-203504 (Patent document 3) Japanese Patent Laid Open No.2003-206481 (Patent document 4) Japanese Patent Laid Open No.2002-363554 (Patent document 5) WO Publication No.2004/029177 A1 pamphlet (Patent document 6) WO Publication No.2004/055910 A1 pamphlet